Bending of coated pipe of relative large diameter presents a number of problems. The coating is bonded to the outer surface of the pipe wall, and when the pipe is bent in conventional bending equipment high compressive force is imposed against the outer surface of the coating at the outside of the bend by the movable bending shoe, which is usually called the strongback. The metal of the pipe is stretched at the outside of the bend and the coating must stretch accordingly if it is to remain properly disposed on the pipe surface. The coating, highly compressed between the strongback and the pipe wall is not free to stretch uniformly. During bending of the pipe, the longitudinal stresses imposed on the coating cause tearing and rupture of the coating, the tears usually being generally arcuate about the lower side of the pipe at the outside of the bend and being spaced along the length of the bend.
It is known that heating of some pipe coatings during pipe bending will reduce or eliminate tearing and rupturing of the coatings, but a satisfactory method for heating the coating on the pipe has not heretofore been found.
One reason that heating of the pipe coating has not with any dependability prevented damage to the pipe coating is that it has not heretofore been realized exactly which area or areas of the coating should be heated prior to bending. Use of resistance heating for preventing damage to the pipe coatings during bending of pipes has never been successfully accomplished, and the prior art is devoid of any specific disclosure relating thereto. Canadian Pat. No. 969,458, and corresponding French Pat. No. 2,276,158, briefly mention that resistance heating might possibly be employed, and that the use of polytetrafluorethylene coatings on the resistance heaters has been envisaged, but these patents do not suggest that resistance heating of pipe coatings, with or without polytetrafluorethylene coatings on the resistance heaters, has ever been accompllished, and do not disclose any guidelines or specifications for doing so, beyond a bare suggestion.
This invention provides methods and apparatus whereby resistance heaters, properly positioned with respect to a bend, may be used successfully for bending coated pipe without damage to the pipe coating on a dependable and consistent basis.